1. Field of the Invention
This invention relates generally to integrated circuits, and more specifically to the layout and use of bond pads and probe pads for testing integrated circuit devices.
2. Related Art
Improvements in semiconductor processing technology have resulted in integrated circuit chips which are more densely populated with microelectronic elements and which provide more functionality than ever before. A chip can be a semiconductor die which is a monolithic structure formed from, for example, silicon or another suitable material.
In the competitive market for integrated circuits, especially for memory devices such as dynamic random access memory (DRAM), it is essential for providers to produce chips which are free of defects. Logic and memory tests are thus performed before a wafer is cut and the die are packaged. A silicon wafer is composed of many separate die. Each die is separated from the adjacent die by a scribe area and each die includes conductive bonding pads. Testing machines typically have a probe card that is lowered to make contact with bonding pads on the wafer in order to test the respective die.
A probe card has multiple needles or pins that lie in a plane with a spacing or pitch between the pins. For the probe pins to make contact with the bonding pads, pressure must be exerted on the pins. If any of the pins is vertically misaligned, additional pressure must be exerted so that all of the pins make contact. The pressure of the pins contacting the surface of the die gouges the bonding pads. Furthermore, in a technique referred to as overdrive, probe pins are made to slide on the surface of the bonding pads in order to remove any aluminum oxide on the surface so that better contact can be achieved. This sliding action results in even larger gouges in the bond pads. Typically, to penetrate the oxide, the probe card and wafer are brought together until the needle probes contact the desired location. The probe card is then “overdriven” a distance which deflects the needle probes and causes them to bend. As the needle probes bend, the ends of the needle probes move horizontally across the bonding pads causing the ends to scrape the surface. This causes the ends to break through the native oxide layer and contact the underlying metal of the bonding pads. The scraping action also displaces some of the metal on the contact location causing a groove and a corresponding ridge.
The gouges in the bond pads resulting from even a single probing operation or “touchdown” weaken subsequent wire bonds to the bond pads. Multiple probing operations and any misalignment of the probe pins can result in severe damage to the bond pads and very poor wire bonding. For most integrated circuits, a wire is bonded to the pad after one or sometimes two touchdowns of the probe pins. However, memory testing may require several touchdowns of the probe pins for numerous reasons including laser repair and subsequent retest.
Small voids can form above the gouges created by one or more touchdowns. These voids between the bond pad and the bond wire create high stress points which may weaken and enlarge over time from thermal cycling, thus resulting in cracks which separate the bond wire from the bond pad. Thermal cycling occurs throughout the life of an integrated circuit device and the failure may occur many years after the initial fabrication and testing.
Semiconductor processing technology is advancing faster than probe card technology. Semiconductor processing technology now allows for a very high density of the integrated circuits and associated bond pads such that, for example, 60 micron bond pads can be spaced with a pitch, or separation between the pads of only 50 microns. Tightly spaced bond pads require probe cards with tightly spaced probe pins, thus the pins must be of a very small diameter. Thinner probe pins are more fragile and can be damaged in the process of testing. Furthermore, probe cards with pins spaced every 50 microns are much more expensive and less durable than standard probe cards with pins spaced about every 100–120 microns.
Another problem with probing the bond pads is that the contact locations on the die necessary for probing are larger than the contact regions needed for wire bonding. In particular, due to the inaccuracies in the x-y placement of the probe pins, the contact locations on the die must be made large enough to accommodate alignment variations between probe cards. This requires that the contact locations be made larger by default, which in turn makes the die larger.